1. Field of the Invention
The present invention relates to a laminated piezoelectric element used in, for example, an automobile fuel injection valve, a precision positioning apparatus of an optical apparatus, a driving element for prevention of vibration or an ink jet printer, and so forth.
2. Description of Related Art
Laminated piezoelectric elements, and particularly laminated piezoelectric elements used in automobile fuel injection valves, are required to ensure reliability over a broad temperature range extending from low temperatures to high temperatures. In environments in which they are subjected to such thermal shock, differences in thermal expansion between the piezoelectric element and external electrode provided on a side of the piezoelectric element cause thermal stress to be applied from the external electrode to the piezoelectric element resulting in the problem of the formation of cracks in the external electrode and the piezoelectric element.
Japanese Unexamined Patent Publications (Kokai) No. 2001-345490 and No. 8-242025 propose a laminated piezoelectric element that improves the durability of the external electrode by providing a plurality of external electrodes having different elongation percentages on a side of the piezoelectric element. In addition, Japanese Unexamined Patent Publication (Kokai) No. 2001-148521 proposes a laminated piezoelectric element that provides an external electrode on a side of the piezoelectric element, a solder layer that is narrower than the external electrode on the external electrode, and an external electrode layer connection lead wire that electrically connects the external electrode on the solder layer. As a result, thermal stress attributable to the difference in thermal expansion between the solder layer and piezoelectric element is not applied directly to the piezoelectric element due to the external electrode being wider than the solder layer.
However, the prior art described in JPP'490 is susceptible to concentration of thermal stress generated in both ends of the external electrodes as the widths of the plurality of external electrodes are the same and, as the thermal stress applied to the piezoelectric element from the external electrodes is large, there was the problem of cracks forming in the external electrodes and piezoelectric element. In addition, in the prior art described in JPP'521, as a solder layer is provided discontinuously relative to the direction of lamination of the piezoelectric, if cracks form at a plurality of locations in the external electrode not in contact with the solder layer due to the difference in thermal expansion between the solder layer and external electrode, sections appear in which there are electrical discontinuities between the locations where cracks have formed. In addition, if the cracks that have formed in the piezoelectric extend to the external electrode not in contact with the solder layer as well, sections similarly appear in which there are electrical discontinuities between the locations where cracks have formed. As a result, as a voltage cannot be applied to a portion of the piezoelectric element and the displacement of the piezoelectric element decreases, there is the problem that the characteristics change during the course of use. In addition, thermal stress is applied to the external electrode and piezoelectric element from both ends of the solder layer in the direction of lamination of the piezoelectric, resulting in the formation of cracks in the piezoelectric element. As the sites where cracks form are piezoelectric active sections and the direction in which the cracks spread is the direction of expansion and contraction of the piezoelectric element, the spreading of the cracks is promoted, thereby resulting in problems in terms of reliability.